Methanol—A Poor Biosignature Gas in Exoplanet Atmospheres

Biosignature gas research has been growing in recent years thanks to next-generation space- and ground-based telescopes. Methanol (CH 3 OH) has many advantages as a biosignature gas candidate. First, CH 3 OH’s hydroxyl group (OH) has a unique spectral feature not present in other anticipated gases in the atmospheres of rocky exoplanets. Second, there are no significant known abiotic CH 3 OH sources on terrestrial planets in the solar system. Third, life on Earth produces CH 3 OH in large quantities. However, despite CH 3 OH’s advantages, we consider it a poor biosignature gas in the atmospheres of terrestrial exoplanets due to the enormous production flux required to reach its detection limit. CH 3 OH’s high water solubility makes it very difficult to accumulate in the atmosphere. For the highly favorable planetary scenario of an exoplanet with an H 2 -dominated atmosphere orbiting an M5V dwarf star, we find that only when the column-averaged mixing ratio of CH 3 OH reaches at least 10 ppm can we detect it with the James Webb Space Telescope (JWST). The CH 3 OH bioproduction flux required to reach the JWST detection threshold of 10 ppm must be of the order of 10 14 molecules cm −2 s −1 , which is roughly three times the annual O 2 production on Earth. Considering that such an enormous flux of CH 3 OH is essentially a massive waste of organic carbon—a major building block of life, we think this flux, while mathematically possible, is likely biologically unattainable. Although CH 3 OH can theoretically accumulate on exoplanets with CO 2 - or N 2 -dominated atmospheres, such planets’ small atmospheric scale heights and weak atmospheric signals put them out of reach for near-term observations.